Removal of silica in the manufacture of alumina.



U ITED STAlES.1 ?ATENT OFFICE.

HARRY cfrnrrnn, OF. EAST ST. LOUIS, I,T:LINOIS, \ASSIGNOR 'IO ALUMINUM COMPANY or AMERICA, or PITTSBURG, PENNSYLVANIA, A CORPORATION or PEN SYLVANIA.

REMOVAL OF SILICA IN THE MANUFACTURE OF ALUMINA.

No Drawing.

Specification of Letters Patent.

Patented Oct. 26, 1909.

Application filed March 5, 1909. Serial No. 431,382.

- To all whom'it may concern:

Be it known that I, HARRY O. PEFFER, a

citizen of the United States, residing at East St. Louis, St. Clair count Illinois, have invented a new and useful Improvement in the Removal of Silica in the Manufacture of Alumina, of which the following is a specification.

My invention relates to the manufacture of alumina, and is, more particularly a modification of, or an improvement u on the wet process, described in the patent of Charles M. Hall, No. 663,167 dated Dec. 4th 1900, its object being further elimination of silica in this process The process described and claimed in the said patent consists, briefly, in the digestion under heat and pressure, of a: mixture of bauxite, carbonate of soda, and caustic lime, with suflicient water. bonate of lime is formed, and the solution of aluminate of soda and. the great bulk of the impurities in the bauxite namely, silica,-

1ron, titanic acid, etc. remain undissolved. It has been found, however, that a small percentage of the silica goes into solution with the aluminate of soda and is .precipitated with it in a subsequent process for precipitation, and thereby contaminates the alumina produced. I have discovered that the small precentage of silica which'remains in solution with aluminate of soda may be precipitated and effectually removed by the following method:

' When a charge is taken from the digester' as described in the above mentioned patent, it consists of a solution of aluminate of soda in water, mixed with a large amount of what is known as red mud. This red mud consists of carbonate of lime with more or less caustic lime, and of the undissolved por tion of the bauxite, consisting of silica, iron, titanic acid, bauxite, and undissolved alumina. It has hitherto been the practice to filter this material at once, se arating the solution of aluminate of soda 2-0111 the red mud. The silica is not, however, removed or separated from this filtered liquor unless it stands fora long time. u V

y invention consistsin allowing the entire charge to stand for a period before filtering. The silica is thereby preci itated, and when the charge is finally filtered, a solution 'In this process, cartime is allowed, the silica which is first prealmost absolutely free from silica is ob- -c'ipitated seems to be re-dissolved. The

charge which comes out of the digesterat the boiling point gradually cools, but is not. completely cooled when filtered. The charge may be agitated and kept in motion, but I have found that the best results are obtained by letting it stand quietly. After standing the prescribed time, or whatever time is found to give the best results, in an particular case, the charge is filtered 1n the usual way, and the solution of aluminate of soda is separated from the solid residuum.

While I have found that a period of six hours gives the best results, I do not limit myself to this period, as with ditferentmix-f tures it may be found that a longer or shorter time will accomplish better results. Neither do I limit myself to a process in which the mixture is allowed to stand quietly, as for commercial reasons it may be better to agitate it.

One method of carring out the process which is commercial y practicable, and which gives good results, is to blow ofl the digester charge into a large tank of sufiicient capacity to. hold a largej number of charges. Additional charges are added to this tank from time to time as the exigencies of the plant require, but the capacity of the tank is so! proportioned to the number of charges that the average time which any charge is allowed to remain. in the tank before removin and filtering is about six hours. In or er to keep the material from settling and making it diflicult to filter, it may be necessary with a large tank and working on- ,a large scale-to agitate the aluminate liquor with the red mud. This may be done by pumping it 01f from'the bottom through a pipe and pumping it into the top, or m other ways which will be obvious to those skilled in the art; 7 What I claim as myinvention and desire to secure byLetters Patent is:

1. The method of eliminating the silica in the process in which aluminate of soda is produced by digesting bauxite in sucha way digester charges to the tank and withdraw- '20 as to produce aluminate of soda, which consists in allowing the aluminate of soda liquor to remain in contact with-the red mud for a suflicient time before filteringto cause the precipitation of the silica.

step which consists in delivering the digester charges to a tank capable of holding a number of such charges, and continually adding ingthe same from time to time, whereby on the average a charge is allowed to remain therein for a 'suflicient time to effect thpre.

cipitation of the contained silica. I

4 The method of eliminating the silica in the process in which aluminate of soda is produced by di esting bauxite in such a way as to produce a luminate of soda, which con: sists in allowing the aluminate of soda liquor to remain in contact with the red mud for about six hours.

5'. In a process for the manufacture of alumina in which bauxite is dissolved to form a solution of aluminate of soda, the step which consists in delivering the digester charges to a tank capable ofholding a number of such charges, and continually adding digester charges to the tank and withdrawing the same from time to time, whereby on the average a charge is allowed to remain therein for about six hours.

6. The method of e'liminating the silica in the process in which aluminate of soda is produced by digesting bauxite in such a way asto produce aluminate of soda, which consists in allowing the aluminate of soda liquor to remain in contact with the red mud for a suflicient time before filtering to cause the precipitation of the silica and then filtering.

7. In a process for the manufacture of alumina in which bauxite is di ested witl lime and carbonate of soda to orm aluminate of soda, the step which consists in allowing the aluminate of soda liquor to remain in contact with the red mud for a suflicient time to cause the precipitation of the silica. and then filtering before the precipitated silica re-dissolves.

In testimony whereof, I have hereunto set my hand.

HARRY C. PEFFER.

'Witnesses: l I EDNA A. RAGLAND, J. M. CHAMBER IN, Jr. 

